For a television signal according to the NTSC system, a video picture is composed with a vertical frequency of about 60 Hz and a horizontal line frequency of about 15.75 KHz. A scanning converter has been proposed which improves the reproduced picture quality by doubling the number of horizontal lines. When this scanning converter is used, the video signal derived therefrom has a vertical frequency of around 60 Hz while it has a horizontal frequency of about 31.5 Khz.
Some computers having a so-called high resolution display are designed to produce a video signal having a line frequency of about 24 KHz. Further, a so-called high definition television (HDTV) system employs a line frequency of about 33.75 KHz.
At present, a multiple scanning type television receiver has been proposed by Sony, as described in more detail in the above-identified co-pending application, which can commonly receive and display various video signals having different line frequencies using a single receiver.
In the proposed multiple scanning type television receiver there is the problem, to be explained in greater detail hereinafter, that in switching between the horizontal and/or vertical frequencies and various video frequencies having different deflecting frequencies, circuit damaging transient voltages are generated in the horizontal deflection circuit. Unless expensive components having wide operating parameters are used, there is the danger that there may be a malfunction.